A light emitting unit of a vehicle lamp uses a semiconductor light emitting element, for example, a Light Emitting Diode (LED), or the like. In this case, a plurality of semiconductor light emitting elements are connected in series in view of the fact that a light amount of a single chip of the semiconductor light emitting element is insufficient. For example, in a vehicle lamp as a headlamp, a light emitting unit in which two LED chips are connected in series is often used.
A Forward Voltage (VF) of a white LED used for a headlamp is approximately 3.5 V per chip, so that a VF of a series-connected LED circuit in the two-chip light emitting unit is approximately 7.0 V. That is, a drive voltage of the LED circuit may be 7.0 V or higher in this case.
Since a battery voltage of an automobile is generally approximately 12 V, in a drive circuit configured to drive the light emitting unit, an input voltage (a battery voltage) is reduced to generate a drive voltage. In order to reduce the battery voltage, for example, a buck switching converter, a series regulator, a current limiting resistor or the like can be used.
Herein, the battery voltage may fluctuate under various conditions, and the battery voltage may drop to approximately 5 V to 6 V in some cases. For example, the input voltage may become lower than 7 V by cranking or the like at the time of engine starting, so that the light amount of the LED sharply decreases. The vehicle lamp is required to maintain a light emitting state even with such a large drop of the battery voltage, so that it is conceivable to adopt a configuration which not only reduces the input voltage but also has a boost function as the drive circuit. However, the number of circuit parts of a buck-boost converter is larger than that of a buck converter and the cost is increased.
In the case of using a buck drive circuit, in order to prevent all LEDs from being turned off due to the drop of the battery voltage, a configuration may be adopted in which a bypass switch is connected in parallel to a part of the LEDs and turned on according to the input voltage dropped to a predetermined value (that is, the part of the LEDs is short-circuited) (for example, see JP-A-2011-162087). Accordingly, even if the input voltage sharply drops, it is possible to maintain a light emitting state of the LEDs other than the part of the LEDs, and it is possible to prevent the LEDs from being all turned off.
However, according to the technique of JP-A-2011-162087, since the light amount of the part of the LEDs sharply decreases at the same time as the input voltage becomes a predetermined value or lower, there is a fear that this is visually recognized as flickering.